The incorporation of chemical species during annealing at the interface between the Ru gate electrode and the dielectric perturbs the electrostatic potential, thus affecting the effective work function of the metal. For both SiO2 and HfO2 gate dielectrics, O-2 anneal at 550 degrees C leads to Ru oxidation (i.e., to RuOx formation) at the Ru/dielectric interface, resulting in a higher effective work function than that of metallic Ru. In turn, when RuOx is exposed to hydrogen, a work function reduction occurs, caused by either a reduction of RuOx into Ru or hydroxyl incorporation at the interface.